OPR computed from levels J = 2, 3,and4 as a function of the excitation temperature T₂₄, comparing PDR observations (symbols) to PDR models (lines). PDR models implement four prescriptions for ortho-para conversion on grains: no conversion (purple), rate equation approach without fluctuations (green), statistical approach with fluctuations (blue), and full conversion (red). Solid lines correspond to models with P/χ = 10⁴, while models with P/χ a factor of 5 above and below this value are shown as dotted lines.